Power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) used for a power control switch, a charge-and-discharge protection circuit switch, etc. of a portable information device, and Insulated Gate Bipolar Transistor (IGBT) are sealed in small surface mounting packages, such as SOP8. For example, the general structure of publicly known SOP8 where power MOSFET was sealed is as follows.
On the die pad part of the lead frame, the semiconductor chip sealed with mold resin is mounted, where the main surface is turned upwards. The back surface of the semiconductor chip constitutes the drain of power MOSFET, and is joined to the die pad part via electroconductive adhesives, such as Ag paste. The source pad connected to the source of power MOSFET and the gate pad connected to the gate electrode are formed in the top layer of the main surface of the semiconductor chip. The source pad is formed in the area wider than the gate pad, in order to reduce the on resistance of power MOSFET. With the same reason, as for the back surface of the semiconductor chip, the whole surface constitutes the drain of power MOSFET.
A plurality of leads which constitute the external connection terminal of SOP8 are exposed to the outside of mold resin. These leads consist of a source lead, a drain lead, and a gate lead. The gate lead is electrically connected with the gate pad by one Au wire, and the source lead is electrically connected with the source pad by a plurality of Au wires. The drain lead is formed in one with the die pad part. The drain lead is electrically connected with the back surface (drain of power MOSFET) of the semiconductor chip mounted on this die pad part.
However, SOP8 of the above structures is difficult to fully lower the source resistance of power MOSFET. Since the cross-section area of the Au wire which connects a source lead with a source pad is small, this is because it is difficult to secure sufficient connection area to a source pad even if the number of an Au wire is increased. In this case, since the size of the silicon chip will also become large when area of the source pad tends to be enlarged and it is going to connect many Au wires, package size will become large. Since much time is needed for the bonding of an Au wire, the problem that the productivity of SOP8 falls is also generated.
Then, technology of connecting a source lead with a source pad is put in practical use in recent years using a metallic ribbon with flexibility which was described, for example to Japanese patent laid-open No. 2004-336043 (Patent Reference 1). Although this metallic ribbon comprises Al foil, Cu foil, etc. whose thickness is about hundreds μm, for example and the width changes also with width of a source pad, generally it is around 1 mm. In order to connect a metallic ribbon to a source pad and a source lead, the wedge-bonding method using supersonic vibration is used.
Since the width of the ribbon is quite large compared with the diameter of an Au wire, there is an advantage of the above-mentioned metallic ribbon in the ability of at least one metallic ribbon to secure sufficient connection area to a source pad. Since a ribbon is constituted from Al cheaper than Au, it is effective in the material cost of SOP8 being reduced.
Japanese patent laid-open No. 2006-196629 (Patent Reference 2) is disclosing the improvement technology of the wedge tool used for connection of the above-mentioned metallic ribbon. A plurality of trenches or notches are formed in the under surface of the wedge tool described in this official report along the direction parallel to the extending direction of the metallic ribbon. Therefore, when this wedge tool is contacted by pressure to the metallic ribbon arranged on a semiconductor chip, a part of under surface of the tool will contact the metallic ribbon. Since excessive ultrasonic vibration energy can be prevented from being transmitted from the wedge tool to the front surface of the semiconductor chip by this, the defect which breakage of a crack, a crack, etc. generates in a semiconductor chip is reduced.
[Patent Reference 1] Japanese patent laid-open No. 2004-336043
[Patent Reference 2] Japanese patent laid-open No. 2006-196629